Mass spectrometer

ABSTRACT

A mass spectrometer which has a toroidal electrostatic field and a uniform magnetic field with non-zero entrance and exit angles, and which is so constructed that the incident end surface of an ion beam on the toroidal electrostatic field defines a concave surface and that the entrance angle and the exit angle of the ion beam relative to the uniform magnetic field are in the positive direction and the negative direction, respectively, thereby to make the elimination of second-order aberrations and axial focusing possible.

United States Patent [191 Taya et al.

[111 3,920,988 Nov. 18, 1975 MASS SPECTROMETER [75] Inventors: Shunroku Taya, Uenohara; Hisashi Matsuda, Takarazuka, both of Japan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: May 7, 1974 21 1 Appl. No.: 467,788

[30] Foreign Application Priority Data May 7 1973 Japan 48-49789 [52] US. Cl 250/296; 250/298 [51] Int. Cl. G0lt 5/00 [58] Field of Search 250/296, 297, 298

[56] References Cited UNITED STATES PATENTS 3,061,720 Ewald 250/296 7/1965 'Ewald et al. 250/296 6/1972 Hull 7. 250/296 Primary Examiner-Harold A. Dixon Attorney, Agent, or Firm-Craig & Antonelli 5 7 ABSTRACT A mass spectrometer which has a toroidal electrostatic field and a uniform magnetic field with non-zero entrance and exit angles, and which is so constructed that the incident end surface of an ion beam on the toroidal electrostatic field defines a concave surface 'and that the entrance angle and the exit angle of the ion beam relative to the uniform magnetic field are in the positive direction and the negative direction. respectively, thereby to make the elimination of secondorder aberrations and axial focusing possible.

3 Claims, 15 Drawing Figures Sheet 1 of 6 US. Patent Nov. 18, 1975 FIG. la PRIOR ART FIG. 2 PRIOR ART US. Patent Nov.,18,1975 Sheet20f6 $920,988

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FIG. I3

L C2H4 RESOLUTION F I6. I4 m/Am=20000u0% VALLEY) J J L l RESOLUTION m/Am 60000 (10% VALLEY MASS SPECTROMETER BACKGROUND YOF'THE- INVENTION 1. Field of the Invention The present invention relatesto improvements in a double focusing mass spectrometer, and'more particularly to the mass spectrometer which diminishes second-order aberrations.

2. Description of the Prior Art H The double focusing type mass spectrometer has heretofore been employed in order to enhance the focusing property and to attain a high resolution. Many of the mass spectrometers combine a cylindrical electrostatic field and a uniform magnetic field.

FIG. 1 is a schematic diagram for explaining an example, in which (a) illustrates the focusing action in the radial (x) direction and (b) that in the axial (y) direction. In the figure numeral 1 designates an ion source, 8 an ion beam, 3 a cylindrical electrostatic field device, 5' a uniform magnetic field device, and 7 a collector. This lens system effects focusing in the radial (x) direction, but it has no focusing in the axial (y) direction. It is, therefore, inferior in the transmission factor of ions.

In order to apply the focusing action in the axial direction, accordingly, a toroidal or spherical electrostatic field device has been used. Alternatively, a nonuniform magnetic field or a uniform magnetic field device with non-zero entrance and exit angles has been utilized. It has been recognized, however, that where the electromagnetic fields having axial focusing action are utilized, the focusing merely in the first-order approximation is unsatisfactory, which necessitates a system of the smallest possible second-order aberrations.

More specifically, a radial width x of a focused image is expressed in the second-order approximation by the following equation:

x=A x +A oz -l-A 8 +A x +A x 01 A p X0 8 Aug (1 A a a 8 0 1111 yo ufi ya Bo pp B0 Here, x,,, a 6 y and 3,, denote the states of an ion beam, x and d the expanding width and angle in the radial direction, respectively, 8,, the energy spread, and

y and B, the expanding width and angle in the axial direction, respectively. Aberration coefficients A, App are determined by an ion optical system. A, denotes the width of the image in the radial direction, and A and A become zero in case of the double focusing. The coefficients of and after A are generally called the second-order coefficients. How small especially those A A 5 and A55 of these second-order coefficients can be made, has been a problem on the ion optical system in the prior art.

For example, H. Hintenberger (Z. Naturforsch. 2a 773 (1957)) has suggested an ion optical system in which, by suitablyv selecting the parameters of a cylindrical electrostatic lensand a uniform magnetic lens with non-zero entrance and exit angles on the basis of numerical calculations, the second-order coefficients A A s andA55 become very small.

I H. W.-W,achsmuth and H. Ewald (Z. Naturforsch. 18a 389 (1963)) have made an experimentfor eliminating the second-order aberrations by the use of a device which comprises in combination a non-uniform magnetic lensand a toroidal electrostatic lens having an electrostatic field constant (to be described later) C 1.75 and in which the entrance and exit end surfaces of an electrode for an ion beam are convex curved surfaces. Further, R. C. Barber, et al, (Rev. Sci. Instrum. 42 l (1971)) have made an experiment for obtaining conditions of small second-order aberrations by the use of a device in which a uniform magnetic lens with nonzero entrance and exit angles is combined with the conventional cylindrical electrostatic lens.

In the theoretical computations for acquiring these ion optical systems, however, there are not considered the influences of fringing fields which are distributed at the entrances and exits of the electrostatic and magnetic fields.

H. Matsuda (Nucl. lnstru. and Meth. 91 637 (I971 has recently made a precise analysis of the fringing fields possible. When the priorart ion optical systems with the corrections of the second-order aberrations taken into consideration were studies by the use of the result, it was revealed that some second-order aberrations remained greatly.

In particular, it was revealed that the second-order aberrations A A g and App for the axial width and angle of the ion beam as have not hitherto been re garded as important remained greatly. These secondorder aberrations appear in such form that an image to be focused into a long rectilinear shape is curved, and they become a cause for degrading the resolution.

The calculations by Matsuda have revealed that these aberrations cannot be made small so long as the cylindrical electrostatic field is used and that they are made small by employing the toroidal electrostatic field which has a curvature not only in the radial direction but also in the axial direction as shown in FIG. 2. In general, the ratio between the radius of an electrostatic lens r and the radius of curvature in the axial direction R is called the constant of the electrostatic lens C A =A5 O Aug A88 Then, FIG. 3 is obtained. In the figure, the axis of abscissas represents C and the axis of ordinates the second-order coefficients, and A,,,,, A p and App are indicated in ratios to the radius of a magnetic lens r,,,. As understood from this figure, the second-order aberrations which amount to App /r,,, E 30 and A p /r,,, 10 in the case of the cylindrical electrostatic lens (C 0) diminish to below l/IO in the vicinity of C 0.5.

In this manner, when the toroidal electrostatic lens is used as the electrostatic lens of the double focusing mass spectrometer, the second-order aberrations can be reduced very sharply. However, where the axial 3 width of a collector slit is expanded in order to increase the quantity of ions, the influences by the bend of the image cannot be removed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a double focusing mass spectrometer in which the second-order aberrations are the smallest, and besides, which has axial focusing action.

Another object is to provide a double focusing mass spectrometer in which the transmission factors of an ion beam within electrostatic and magnetic fields are large.

Still another object is to provide a double focusing mass spectrometer of high resolution and high sensitivity.

Yet another object is to provide a double focusing mass spectrometer in which, even when the expanding angle of an ion beam or the energy spread is large, the resolution does not become lower.

The present invention for accomplishing these objects is characterized by a toroidal electrostatic lens in which an electrode edge on the entrance side of an ion beam forms a concave surface, and a uniform magnetic lens with non-zero entrance and exit angles. in which the entrance angle of the ion beam is in the positive direction (a case where the angle defined between the plane normal to the incident beam and the plane of magnetic poles is measured clockwise from the former plane is made the positive direction) while the exit angle is in the negative direction (a case where the angle defined between the plane normal to the emergent beam and the plane of the magnetic poles is measured clockwise from the former plane is made the negative direction).

A further characterizing feature is that the parameters of various parts lie in the following ranges:

Ratio between the electrostatic lens radius and the magnetic lens radius: 1.0 r-,./r,,, 1.1

Deflection angle of the electrostatic lens: 83 I.

Deflection angle of the magnetic lens: 85 I' Constant of the electrostatic lens: 0.46

Radius of curvature of the concave of the electrode on the entrance side: 0.4 r, p 0.6 r, Entrance angle of the uniform magnetic lens: 28

Exit angle of the uniform magnetic lens: 8 6

Distance from an ion source to the entrance of the electrostatic lens: 1.2 r,, 1,, 5: 1.3 r,,

By adopting such a construction, it becomes possible to provide a double focusing mass spectrometer which renders all the second-order aberrations the least and which has the axial focusing action.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood by reference to the following detailed description and to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram for explaining the radial focusing (a) and the axial focusing (b) of a prior art double focusing mass spectrometer which is based on the combination between a cylindrical electrostatic field and a uniform magnetic field;

FIG. 2 is a perspective view ofa toroidal electrostatic field arrangement;

FIG. 3 is a diagram showing the relations of the second-order aberrations to the constant C of a toroidal electrostatic lens and the width and expanding angle thereof in the axial direction;

FIG. 4 is a schematic view for explaining a double focusing mass spectrometer according to the present invention;

FIG. 5 is a schematic view showing the axial focusing of the device of the present invention;

FIGS. 6 to 11 are diagrams each showing the relations between the parameters and the second-order aberrations of the device of the present invention;

FIG. 12 is a schematic view, partly in section, of the essential portions of an embodiment of the mass spectrometer according to the present invention; and

FIGS. 13 and 14 are spectral diagrams obtained with the mass spectrometer of the present invention, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The outline of a device based on the present invention is illustrated in FIGS 4 and 5. FIG. 4 shows the focusing action in the radial (.r) direction, while FIG. 5 shows the focusing in the axial (y) direction. In the figures numeral 1 designates an ion source, 2 a slit of the ion source, 3 a toroidal electrostatic lens, 4 a shield electrode, 5 a uniform magnetic lens with non-zero entrance and exit angles, 6 a collector slit, 7 a detector, and 8 an ion beam.

With such construction, in the radial direction as seen in FIG. 4, the ion beam 8 drawn out from the ion source 1 is subjected to the directional focusing as a part A in the figure by means of the toroidal electrostatic lens 3. After subsequently passing through the magnetic lens 5 with non-zero entrance and exit angles, the ion beam is subjected to the double focusing (the directional focusing and the velocity focusing) at the collector slit 6 (a part B in the figure). On the other hand," the focusing action in the axial direction is as seen in FIG. 5. The ion beam 8 (the ion beam expanding from one point of the ion source 1) is focused on a part C near the exit of the magnetic lens 5 with nonzero entrance and exit angles, while a beam 8' emerging from the ion source 1 in parallel is focused on a part D near the entrance of the magnetic lens 5. The axial width of the ion beam in the magnetic lens is accordingly reduced considerably. In consequence, the transmission factor of the ion beam becomes larger in comparison with the prior art device having no axial focusing, such as the double focusing mass spectrometer which comprises the cylindrical electrostatic lens and the uniform magnetic lens in combination. The parameters of the device are determined so that all the second-order aberrations may become the least at the same time.

FIG. 12 shows a schematic view, partly in section, of the essential portions of an embodiment in which r,,, 200 mm. In the figure numeral 1 indicates an ion source, 2 an ion source slit, 3 a toroidal electrostatic lens, 9 an a-slit, 10 a B-slit, 11 an ion monitor, 5 a mag: netic lens with non zero entrance and exit angles, 6 a collector slit, and 7 a collector composed of a multiplier. The parameters of the device are determined as follows:

Toroidal electrostatic field;

(1),, (deflection angle of the electrostatic lens) Q1 85.2

C, (constant of the electrostatic field) 0.5

r,,/r (where r, denotes the radius of the electrostatic lens and r,,, the radiusofthe magnetic l'ens l .06

p (radius of the concave curvature of an electrode on the entrance side) 0.5 r, I

Uniform magnetic field with'non-z ero entrance and exit angles; v v 4 (la (deflection angle of the magnetic lens) 90 e (entrance angle of the magnetic lens) 30 5 (exit angle of the magnetic lens) Slit positions;

1,, (distance between the ion source slit and the electrode plane of the electrostatic lens on the entrance side) 1.24 r,

1,, (distance between the part A and the electrode plane of the electrostatic lens on the exit side) 0.335 r,,

1,, (distance between the part A' and the incident point of the ion beam to the magnetic lens) 0.845

1,," (distance between the collector slit and the emergent point of the ion beam from the magnetic lens) 1.363 r t Further, regarding the expansion of the 'ion beam after emerging from the ion source, radial width x 10 u radial angle a 0.003 rad, energy spread 8,, 0.001, axial width y 1 mm and axial angle B 0.001 rad.

The second-order aberrations in the ion optical system thus constructed as calculated in microns are as follows:

A X? 0.001 um, A x,, a,, =0.103 um, Awe, 8 =0.00l um, A a =-0.000 um, A I a, 8 0.003 um, A 58,, -0.000 pm, A y, 0.493 pm, A g y, B, 0.483 um, A55 3, O.277 pm.

As apparent from these values, all the second-order aberrations which were several tens pm in the prior art device become less than 0.5 pm.

Measured results obtained with the device of the embodiment are illustrated in FIGS. 13 and 14. The results in FIG. 13 were obtained under the state under which neither the a-slit nor the B-slit was mounted. The mass differences of CO and N were 12 milli mass, and a high resolution above 20,000 was obtained in a 10% valley from this degree of separation. The spectrum in FIG. 14 was obtained when the resolution adjustment was made by mounting the a-slit and the ,B-slit. The resolution in this case was 60,000 in the 10% valley. Since,

with the prior art device as shown in FIG. 1, the resolution is approximately 20,000 at r 200 mm, the performance which is about three times higher is acquired with the mass spectrometer of the present invention.

As to the transmission in the device, 80% of the incident ions was measured by the ion monitor ll. Further, where the ion beam passed through the magnetic lens, 50% of the ions incident on the magnetic lens was measured by the collector portion. In contrast, with the prior art device as illustrated in FIG. 1, the former value is 40% and the latter value is 2%. It is, accordingly, understood that the mass spectrometer of the present invention is about times higher in transmission than the prior art device.

FIGS. 6 to 11 illustrate to what extent the secondorder aberrations change depending on the values of the parameters of the device.

FIG. 6 depicts the relationship between the ratio of the orbital radii of the electrostatic and magnetic lenses (r /r and the second-order aberrations, FIG. 7 the relationship between the deflection angle of the electrostatic lens and the second-order aberrations, FIG. 8 the relationship between the toroidal electrostatic lens constant (C and the second-order aberrations, FIG. 9 the relationship between the concave edge of the electrostatic lens on the entrance side (r,,/d and the second-order aberrations, FIG. 10 the relationship between the exit angle from the magnetic lens (6 and the second-order aberrations (e, 30). and FIG. 11 the relationship between the distance from the ion source to the entrance of the electrostatic lens (1,) and the second-order aberrations.

These graphs represent the magnitudes of the second-order aberrations in the case where only one parameter is varied and where the other parameters are fixed at the values mentioned previously.

From these graphs, it is understood that the secondorder aberrations diminish. Especially, the second- As seen from the experimental results stated above and theoretical calculations, in accordance with the present invention, the second-order aberrations are satisfactorily removed. Therefore, even when the angle and energy spread of the ion beam are large, the resolution does not lower, and the ion current value becomes large to that extent. Furthermore, since the axial focusing is also effected, the transmission factor increases. In consequence of these results, where the requisites of the parameters mentioned in the summary of the invention are fulfilled, the second-order aberrations are eliminated and besides the axial focusing action is had, and the expected objects and advantages can be achieved.

As described above, the mass spectrometer of the present invention reduces the various aberration coefficients, particularly the second-order aberration coefficients, at the same time, and therewith, it enhances the transmission factors of the ion beam within the electrostatic and magnetic lenses. Thus, the invention makes a high-resolution and high-sensitivity mass spectrometer possible, and is very greatly effective in industry.

While the invention has been described by reference to particular embodiments, thereof, it will be understood that numerous and further modifications may be made by those skilled in the art without actually departing from the invention. We, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What is claimed is:

1. In a mass spectrometer including means generating an ion beam, a toroidal electrostatic field device and a uniform magnetic field device with end portions disposed with respect to said ion beam to provide nonzero entrance and exit angles, the improvement characterized in that an edge of said toroidal electrostatic field device on the entrance side of the ion beam forms a concave surface, and that the entrance angle into said uniform magnetic field device is oriented in the positive direction while the exit angle therefrom is oriented in the negative direction, whereby the elimination of se- 7 cond-order aberrations and focusing in the axial direction are made possible.

2. The mass spectrometer according to claim 1 characterized in that the parameters of various parts lie in the following ranges:

ratio between the electrostatic lens radius r,, and the magnetic lens radius r,,, is 1.0 S r S 1.1;

deflection angle I of the electrostatic lens is 83 deflection angle I', of the magnetic lens is 85 I',

95; constant C, of the electrostatic field is 0.46 5 C radius of curvature p of the concave of the electrode on the entrance side is 0.4 r,, 5 p 0.6 r,,;

entrance and exit angles 6 and 6 of the uniform magnetic lens are 28 e 33 and 8 6 l 2; and

distance 1,, from an ion source to the entrance of the electrostatic lens is 1.2 r, 5 l 5 1.3 r,,,; whereby the elimination of second-order aberrations and the focusing in the axial direction are made possible.

3. The mass spectrometer according to claim 1 characterized in that the parameters of various parts have the following values:

toroidal electrostatic field;

d), (deflection angle of the electrostatic lens) 85.2;

C (constant of the electrostatic field) 0.5;

r /r (r denotes the electrostatic lens radius and r the magnetic lens radius) 1.06,

p (radius of curvature of the concave of the electrode on the entrance side) 0.5 r,

uniform magnetic field with non-zero entrance and exit angles;

4), (deflection angle of the magnetic lens) 5, (entrance angle into the magnetic lens) 30,

e (exit angle from the magnetic lens) =l0, slit position,

1;, (distance between an ion source slit and the electrostatic lens electrode on the entrance side) 1,," (distance between the electrode plane of the electrostatic lens on the exit side and a part A) 0.335

l,,, (distance between the part A and the entrance point of the ion beam into the magnetic lens) 0.845 r,,,, and

1 (distance between the exit point of the ion beam from the magnetic lens and a collector slit) 1.363 r,,,, whereby the elimination of second-order aberrations and the focusing in the axial direction are made possible. 

1. In a mass spectrometer including means generating an ion beam, a toroidal electrostatic field device and a uniform magnetic field device with end portions disposed with respect to said ion beam to provide non-zero entrance and exit angles, the improvement characterized in that an edge of said toroidal electrostatic field device on the entrance side of the ion beam forms a concave surface, and that the entrance angle into said uniform magnetic field device is oriented in the positive direction while the exit angle therefrom is oriented in the negative direction, whereby the elimination of second-order aberrations and focusing in the axial direction are made possible.
 2. The mass spectrometer according to claim 1 characterized in that the parameters of various parts lie in the following ranges: ratio between the electrostatic lens radius re and the magnetic lens radius rm is 1.0 < or = re/rm < or = 1.1; deflection angle Psi e of the electrostatic lens is 83* < or = Psi e < or = 88*; deflection angle Psi m of the magnetic lens is 85* < Psi m < 95*; constant C1 of the electrostatic field is 0.46 < or = C1 < or = 0.65; radius of curvature Rho 1 of the concave of the electrode on the entrance side is 0.4 re < or = Rho 1 < or = 0.6 re; entrance and exit angles epsilon 1 and epsilon 2 of the uniform magnetic lens are 28* < or = epsilon 1 < or = 33* and -8* < or = epsilon 2 < or = -12*; and distance lo'' from an ion source to the entrance of the electrostatic lens is 1.2 rm < or = lo'' < or = 1.3 rm; whereby the elimination of second-order aberrations and the focusing in the axial direction are made possible.
 3. The mass spectrometer according to claim 1 characterized in that the parameters of various parts have the following values: toroidal electrostatic field; phi e (deflection angle of the electrostatic lens) 85.2*; C1 (constant of the electrostatic field) 0.5; re/rm (re denotes the electrostatic lens radius and rm the magnetic lens radius) 1.06, Rho 1 (radius of curvature of the concave of the electrode on the entrance side) 0.5 re, uniform magnetic field with non-zero entrance and exit angles; phi m (deflection angle of the magnetic lens) 90*, epsilon 1 (entrance angle into the magnetic lens) 30*, epsilon 2 (exit angle from the magnetic lens) -10*, slit position, le'' (distance between an ion source slit and the electrostatic lens electrode on the entrance side) 1.24 rm, le'''' (distance between the electrode plane of the electrostatic lens on the exit side and a part A) 0.335 rm, lm'' (distance between the part A and the entrance point of the ion beam into the magnetic lens) 0.845 rm, and lm'''' (distance between the exit point of the ion beam from the magnetic lens and a collector slit) 1.363 rm, whereby the elimination of second-order aberrations and the focusing in the axial direction are made possible. 